Japanese Patent Application Laid-open No. 4-47104 discloses a Rankine cycle system in which an expander is operated by a vapor generated by an evaporator, and the vapor discharged from the expander is liquefied in a condenser and returned to the evaporator, wherein a valve provided in an entrance of the expander is opened and closed according to the magnitude of energy of the vapor generated by the evaporator so as to control the timing with which the vapor is supplied to the expander, thus ensuring a maximum output torque.
Furthermore, Japanese Patent Application Laid-open No. 58-48706 discloses a Rankine cycle system in which an expander is operated by a vapor generated by an evaporator, and the vapor discharged from the expander is liquefied in a condenser and returned to the evaporator, wherein when the vapor introduction pressure of the condenser is higher than the vapor discharge pressure of the expander, a bypass passage is opened so as to reduce overexpansion loss of the expander, the bypass passage providing a connection between the entrance side of the condenser and a position of the expander immediately before the position where expansion is completed.
Moreover, Japanese Patent Application Laid-open No. 61-87990 discloses a vane-type compressor in which a rotary valve is provided on a rotating shaft of a rotor supporting vanes, the rotary valve controlling intake and exhaust of a vane chamber, and the intake timing and the exhaust timing of the rotary valve are made variable.
For example, with regard to a Rankine cycle system in which water is heated by an evaporator that carries out heat-exchange with exhaust gas of an internal combustion engine to generate a vapor, the vapor operates a displacement-type expander so as to produce a shaft output, and the vapor discharged from the expander is turned back into water by a condenser and supplied again to the evaporator, the pressure and temperature of the vapor supplied from the evaporator to the expander are preset at rated values according to the performance of the expander, and the temperature of the vapor discharged from the expander into the condenser is preset at a rated value according to the performance of the condenser. However, the pressure and temperature of the vapor generated in the evaporator vary according to the transient state of the evaporator, the operational state of the internal combustion engine, the amount of water supplied to the evaporator, etc., and the vapor pressure and temperature at which the condenser can exhibit maximum performance also vary according to the transient state of the condenser, the cooling state of the condenser (temperature of external air, rotational speed of a cooling fan, strength of air flow), etc.
In FIG. 21A, the ordinate and the abscissa denote a pressure p and a specific volume v of vapor respectively, and when a vapor which has a rated pressure p1 at the entrance of the expander expands within the expander by a set expansion ratio ε, which has been set in advance, and the pressure at the exit of the expander changes from this p1 to a rated value p2, the expander and the condenser can exhibit maximum performance. However, as hereinbefore described, the pressure at the entrance of the expander varies due to various factors, and the pressure at the exit of the expander at which the expander and the condenser can exhibit maximum performance also varies due to various factors. There is therefore a possibility that the pressure at the exit of the expander might not coincide with the pressure at which the expander and the condenser can exhibit maximum performance, thus preventing the expander and the condenser from exhibiting satisfactory performance.
That is, as shown in FIG. 21B, in the case where, even if the expansion ratio coincides with the set expansion ratio ε, the pressure at the entrance of the expander is p1′, which is much larger than the rated value p1, the pressure at the exit of the expander becomes higher than the rated value p2, vapor that still has energy to drive the expander is wastefully discharged, the performance of the expander cannot be exhibited satisfactorily and, moreover, the load imposed on the condenser increases, thus degrading the condensation performance, which is a problem. On the other hand, as shown in FIG. 21C, in the case where, even if the expansion ratio coincides with the set expansion ratio ε, the pressure at the entrance of the expander is p1′, which is much smaller than the rated value p1, since the pressure at the exit of the expander becomes lower than the rated value p2, the vapor performs negative work within the expander, thus reducing the output, which is a problem.
Such problems similarly occur in the case where the temperature at the entrance of the expander is higher or lower than the rated value, in the case where the leak rate of vapor within the expander is large or small, or in the case where the pressure at the exit of the expander at which the expander and the condenser can exhibit maximum performance has changed from the rated value p2 due to various factors.